I tried downloading a song from itunes only half the song was able to record on the computer any ideas to finish it? thanks

Depending upon what country that you are in (music can't be re-downloaded in all countries) then try deleting the incomplete track from your iTunes library and redownload it via the Purchased link under Quick Links on the right-hand side of the iTunes store home page on your computer's iTunes : re-downloading.

If you aren't in a country where you can re-download music or if it re-downloads in the same state then try the 'report a problem' link from your purchase history : log into your account on your computer's iTunes via Store > View My Account (Store > View My Apple ID on iTunes 11) and you should then see a Purchase History section with a 'see all' link to the right of it ; click on that and you should see a list of your purchases ; find that song and use the 'Report a Problem' link and fill in details about the problem (iTunes support should reply within, I think, about 24 hours).

Some people have had a problem with the 'report a problem' link (it's been taking people to this site on a browser instead of showing a form in iTunes) - if it does that to you then try contacting iTunes support via this page : http://www.apple.com/support/itunes/contact/- click on Contact iTunes Store Support on the right-hand side of the page.